AuthorTopic: U67: Add More Top End? (Read 17089 times)

Polystyrene film caps are harder to get these days since all but one of the polystyrene film makers closed shop. The only Amercan brand still making them is Reliable Capacitors in Santa Fe Springs, CA. Their limit is 1000 pf. Some asian caps are still available.

In a pinch a good sub would be a quality polypropylene film and foil cap like the WIMA FKP-2 series. Those come in 1000 volt ratings, 100, 68, 47 and 33 pf are available values.

Polystyrene film caps are harder to get these days since all but one of the polystyrene film makers closed shop. The only Amercan brand still making them is Reliable Capacitors in Santa Fe Springs, CA. Their limit is 1000 pf. Some asian caps are still available.

In a pinch a good sub would be a quality polypropylene film and foil cap like the WIMA FKP-2 series. Those come in 1000 volt ratings, 100, 68, 47 and 33 pf are available values.

Do you have an opinion on the performance of teflon-dielectric capacitors as a substitute for polystyrene? The obvious downside being cost.

Bass Lim's Rel-Cap made me some Teflon film & foil capacitors in the 1mfd range, and, hoping that higher cost would tranlate to higher utility, I did not find them particularly musical or better in resolution or other relevant parameters than tin film & foil, or most intriguing, but very hard to make and find: copper film & foil.

It is also worth repeating what I wrote a while ago: different capacitances demand different materials in the audio chain. For example, I've found that in the capacitance range of 10.000pf or lower nothing beats styrene. But styrene's advantages and sonic attraction diminishes with values above that.

Neumann's NG 1 power supplies for U47 did feature chokes (through 1955), then dropped them, and within a couple of years, dropped chokes from all its tube mic power supplies. I guess, they thought it overkill, and not worth the extra expenses?

For example the initial U67u 00-00-00S. (Considering that 17 is the highest number for a capacitor in this model, the designation as C17 may be an indication that is may have been an afterthought...):

I'm curious about the omission of C-17 as shown in the schematic posted by Uwe (U67u). My understanding is that removing C-17 was a common modification and seems rather controversial.What effect does this have on the sound?The other differences in these two schematics are R4, R5 and R19.

Reply #8 goes a bit deeper into the history of the (short lived) omission of C17.

The mic is too bright without C17. Removing C17 does not "restore" full frequency response of capsule or mic amp to a balanced output. The capacitor removes the 9dB boost at 9KHz the capsule would otherwise produce.

Polystyrene film caps are harder to get these days since all but one of the polystyrene film makers closed shop. The only Amercan brand still making them is Reliable Capacitors in Santa Fe Springs, CA. Their limit is 1000 pf. Some asian caps are still available.

In a pinch a good sub would be a quality polypropylene film and foil cap like the WIMA FKP-2 series. Those come in 1000 volt ratings, 100, 68, 47 and 33 pf are available values.

I commissioned Bass Lim, in charge at Relcap, to roll me a few 1.0 mfd styrenes, so I speak from direct experience:Not only do styrene caps sound lousy as coupling caps in values above 10.000mfd, they also would be so large at 1.0mdf (the standard in condenser mics) that they would not fit inside a U67.

Tin film & foil is the way to go, and even there, the specialty under-sized cap Relcap makes for me would not fit a U67 unless I reposition the tube and its socket (PITA).

Wima FKP3 are available in much lower voltages and therefore smaller sizes.Depending on the purpose these may be a good option, even paralleled for higher capacitance values.They have the lowest distortion of all available (I built my rig now and can measure down to -154 dB or 0.000002 %).Selected specimen come down to this figure, even unselected they're not too much worse.The downside: they are really hard to find if you don't want to buy really large amounts.

Of course, if you want a cap to deliver a special "sound" others might be the choice.